I used to do that...but I became obsessed with the idea, and at some point found myself driving around with a trunk full of everything I could think of (extra belt, sections of hose in different sizes, clamps, plug wires, plugs, caps, rotors, module, tools, meters, fluids etc)...looked like rolling shop/parts store in the trunk, and still when something goes wrong it's never what you have on board...
Murphy's Law #21 I think...
My trunk junk now consists of
A high quality 45 pc ratchet set.
A few extra torx and allen sockets.
A small set of hand tools (pliers, vice grip, screwdrivers, hammer, combo wrenches. etc)
Commercial grade jumper cables.
A small fire extinguisher.
A DVOM and an extra battery for it.
Some road flares (gonna replace with LED strobes soon)
Heavy duty tow strap.
Pro quality 4 way lug wrench.
Large folded laminated cardboard slab to kneel/lay on instead of the ground...
There's other stuff, I can't think of it at the moment...let's say I'm more prepared than the average joe six-pack
Many people have been glad I happened along when they had trouble...
I don't mind, it comes back to you when you are in need.